Method for separating iron from an organic phase containing uranium and method for extracting uranium from an aqueous solution of mineral acid containing uranium and iron

ABSTRACT

The application relates to a method for separating iron from an initial liquid organic phase containing uranium and iron, wherein the initial liquid organic phase is contacted with an aqueous solution referred to as aqueous de-ironing solution, whereby the iron passes into the aqueous solution to form a final liquid aqueous phase, and uranium remains in the initial liquid organic phase to form a final liquid organic phase referred to as de-ironed organic phase. The method is characterised in that the aqueous de-ironing solution contains an inorganic acid and uranium, and does not contain iron. The application also relates to a method for extracting uranium from an aqueous solution of an inorganic acid containing uranium and iron.

TECHNICAL FIELD

The invention relates to a method for separating iron, from a liquid organic phase containing uranium and iron.

More precisely, the invention relates to a method for separating iron, from a liquid organic phase containing uranium and iron.

The invention applies to the separation of iron from a uranium (uranium bearing, “uranifère”) liquid organic phase, containing an organic extraction system comprising an organic extractant diluted in an organic diluent.

This organic phase may be notably an organic phase resulting from the extraction of uranium by a solvent from an aqueous uranium bearing solution of mineral, inorganic acid, such as phosphoric acid, nitric acid or sulphuric acid.

The invention thus further relates to a method for extracting uranium from an aqueous solution of mineral acid, containing uranium and iron.

This aqueous solution of mineral acid may equally well be an aqueous uranium bearing solution of phosphoric acid, such as industrial phosphoric acid, derived from the lixiviation, attack, of a natural phosphate ore, generally based on apatite, by sulphuric acid, as an aqueous uranium bearing solution of sulphuric acid or nitric acid derived from the lixiviation, attack, of a non-phosphate uranium bearing ore, for example non-apatite based, by sulphuric acid or nitric acid. The invention thus finds application in the treatment of natural phosphates to beneficiate the uranium that these phosphates contain, but also in the treatment of uranium bearing ores subjected to an attack, lixiviation, by sulphuric acid or nitric acid in order to beneficiate the uranium present in these ores.

STATE OF THE PRIOR ART

If a focus is firstly made on the beneficiation of uranium contained in phosphates, it is firstly necessary to recall that to meet the increasing demand for uranium, notably for nuclear reactors, it has today become interesting to beneficiate the uranium contained in so-called “non-conventional” sources such as phosphates.

Uranium is in fact present at very low concentrations in phosphates, generally from 50 to 200 ppm. Certain phosphate deposits may contain non-negligible quantities of uranium and thus become potentially exploitable uranium deposits.

The beneficiation of uranium from phosphates firstly relates to the beneficiation of the uranium contained in industrial phosphoric acid, referred to as “wet process” phosphoric acid, which constitutes, with phosphate fertilizers, the main production from phosphates. This “wet process” phosphoric acid is the acid obtained by attack of natural phosphate ores by concentrated sulphuric acid, followed by a solid-liquid separation treatment for separating the phosphoric acid from the gypsum that has precipitated during the attack.

More exactly, at the end of the attack by sulphuric acid, a solution of phosphoric acid is obtained which titrates for example between 26% and 32% by weight of P₂O₅ in the case of a so-called “Dihydrate” method, which is the method generally implemented in current production installations.

This solution of phosphoric acid contains, apart from uranium, already cited, significant impurities at the forefront of which stands iron, but also silica, vanadium, molybdenum, and zirconium.

In order to recover uranium from these aqueous solutions of phosphoric acid, the extraction of uranium is carried out by an organic solvent comprising an organic extractant in an organic diluent and a uranium bearing organic phase is thereby obtained.

Yet, this uranium bearing organic phase also contains the impurities listed above, and mainly iron which is extremely bothersome and does not make it possible to obtain, during the following step of back extraction (“désextraction”), uranium having the required purity with a view to its later use.

Indeed, non-negligible quantities of iron are extracted despite a high separation factor between uranium and iron (namely: FS_(U/Fe) close to 200), which can bring about the formation of precipitates during the back extraction of uranium in carbonate medium.

As an example, iron precipitates in the form of iron hydroxide during the step of re-extraction (“réextraction”) of uranium, which requires additional filtration operations and poses problems with regard to carrying out the method.

A co-extraction of impurities such as iron and phosphates is disadvantageous because it makes it difficult to comply with the ASTM specifications on uranium bearing concentrates.

The beneficiation of uranium contained in phosphates, and more specifically in the aqueous solutions of phosphoric acid derived from the attack of phosphate ores by sulphuric acid, has been the subject of numerous studies.

The document FR-A-2 596 383 [1] and the document EP-A1-0 239 501 [2] describe in a general manner a method for extracting uranium present in solutions of phosphoric acid, notably in solutions of phosphoric acid obtained from phosphate ores containing iron.

The method of these documents uses novel extractant molecules or, more exactly, a novel synergistic mixture, implemented in a single cycle of extraction/re-extraction of uranium, which increases the partition coefficient of uranium, and comprises a step of selective de-ironing (iron removal, deferrization) of the solvent by an acid upstream of the step of back extraction of uranium.

This acid may be selected from oxalic acid, a mixture of phosphoric and sulphuric acid, and de-ironed (iron-removed, deferrization) phosphoric acid.

This acid prevents phenomena of precipitation of ferric hydroxides during the back extraction of uranium.

Thus, more specifically, documents FR-A-2 596 383 [1] and EP-A1-0 239 501 [2] describe a method for separating iron from an organic uranium bearing solution in which a system of extractants constituted by a neutral phosphine oxide and an acid organophosphorous compound is used.

The novel extractant molecules used in the methods of documents FR-A-2 596 383 [1] and EP-A1-0 239 501 [2] are notably those described in documents FR-A-2442 796, FR-A-2 459 205, FR-A-2 494 258, and EP-A1-053 054.

Although promising, this method has several major drawbacks:

-   -   the novel synergistic extractant mixture used increases the         partition coefficients of uranium and iron compared to         conventional solvents, but uranium/iron selectivity is less         good;     -   the operating expenses turn out to be higher whatever the acid         chosen for selective de-ironing of the solvent, notably on         account of industrial phosphoric acid losses, of the impact of         potential recycling at the step of lixiviation of the ore, and         of losses of reagent.

More exactly, if industrial de-ironed phosphoric acid is used, then it is necessary to add mixer-decanter stages to de-iron the acid (to remove iron from the acid, to deferrize the acid); if a mixture of phosphoric acid and sulphuric acid is used, this has an impact on the step of lixiviation of the ore, and a contamination of industrial phosphoric acid takes place; and if oxalic acid is used, the cost of this reagent is high, and the reagent regeneration yield is insufficient.

One means for improving the extraction of uranium from an aqueous solution of phosphoric acid consists in replacing the synergistic mixture Di₂EHPA/TOPO by combining the two functions “cationic exchanger” and “solvating extractant” within a single and same compound.

A bifunctional extractant notably has the advantage of there being only a single compound instead of two to manage.

The document FR-A1-2,604,919 [3] relates to a bifunctional compound comprising a phosphine oxide function and a phosphoric or thiophosphoric function, these two functions being linked to one another by an appropriate spacer group, such as an ether, thioether, polyether or polythioether group.

This type of compound has two drawbacks. Indeed, tests that have been carried out with one of these compounds have shown that, if this compound is solubilised in n-dodecane, a third phase is formed during the extraction of uranium, whereas, if it is solubilised in chloroform, a third phase is also formed but during the back extraction of uranium. Yet, the appearance of a third phase is totally unacceptable for a method intended to be implemented at an industrial scale. Furthermore, the presence within the spacer group of a P—O or P—S bond, which is easily hydrolysable, makes these compounds extremely sensitive to hydrolysis.

It may be considered that, in this document, the organic phase produced at the end of the extraction contains iron, and no method for de-ironing of this phase is proposed.

The document WO-A1-2013/167516 [4] deals with bifunctional compounds that are free of the various drawbacks exhibited by the bifunctional compounds proposed in the aforesaid documents [2] to [3] and, in particular, of the necessity of reducing beforehand uranium(VI) into uranium(IV), of the formation of a third phase and of the risk of hydrolysis.

The bifunctional compounds of this document correspond to the following general formula (I):

wherein: m is a whole number equal to 0, 1 or 2; R¹ and R², identical or different, are a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms;

R³ is:

-   -   a hydrogen atom;     -   a hydrocarbon group, saturated or unsaturated, linear or         branched, comprising from 1 to 12 carbon atoms and optionally         one or more heteroatoms;     -   a monocyclic hydrocarbon group, saturated or unsaturated,         comprising from 3 to 8 carbon atoms and optionally one or more         heteroatoms; or     -   a monocyclic aryl or heteroaryl group;         or instead R² and R³ together form a —(CH₂)_(n)— group wherein n         is a whole number ranging from 1 to 4;         R⁴ is a hydrogen atom, a hydrocarbon group, saturated or         unsaturated, linear or branched, comprising from 2 to 8 carbon         atoms, or a monocyclic aromatic group; whereas         R⁵ is a hydrogen atom or a hydrocarbon group, saturated or         unsaturated, linear or branched, comprising from 1 to 12 carbon         atoms.

It may be considered that, in this document, the organic phase produced at the end of the extraction contains iron, and no method for de-ironing of this phase is proposed.

Although these novel extractant molecules make it possible to do without a synergistic mixture of extractants and although they are more efficient in terms of uranium extraction, they are however not sufficiently selective to enable the elimination of the step of de-ironing of the solvent in a method for beneficiating uranium contained in uranium bearing phosphate ores.

If the focus is now placed on the beneficiation of uranium contained in non-phosphate uranium bearing ores, the starting point is carrying out an attack, lixiviation, of these ores with sulphuric acid or nitric acid by means of which an aqueous uranium bearing solution of sulphuric acid or nitric acid is obtained.

This aqueous uranium bearing solution of sulphuric acid or nitric acid contains, apart from uranium, notable impurities that were present in the ore, at the forefront of which stands iron, but also silica, vanadium, molybdenum, and zirconium.

In order to recover uranium from these aqueous solutions of sulphuric acid or nitric acid, the extraction of uranium is carried out by an organic solvent comprising an organic extractant in an organic diluent and a uranium bearing organic phase is thereby obtained.

Yet, this uranium bearing organic phase, just like the organic phase derived from an aqueous uranium bearing solution of phosphoric acid, also contains the impurities listed above, and mainly iron, and this is extremely bothersome and does not make it possible to obtain, during the following step of back extraction, uranium having the required purity in view of its later use.

Numerous methods were developed in the years 1950-1960 in order to recover uranium quantitatively and selectively from sulphuric liquors.

The method referred to as the DAPEX method may thus be cited, which is based on the mixture of extractants Di₂EHPA/TBP. The main constraint of this method is its sensitivity to Fe(III) ions.

This has led to the replacement of this method in industrial installations by the method referred to as the AMEX method described in the document of Merritt, R. C.: “The Extractive Metallurgy of Uranium”, Colorado School of Mines Research Institute (1971) [5], which is based on a mixture of a tertiary amine, for example a tri-octyl/dodecyl amine insoluble in water, and tridecanol playing the role of phase modifier. This mixture is highly selective vis-à-vis iron.

It may be considered that, in this document, the organic phase produced at the end of the extraction contains iron, and no method for de-ironing of this phase is proposed.

There thus exists, in light of the above, a need for a method for separating iron (that is to say a de-ironing method (a deferrizing, deferrization, method, a method for removing iron) from a liquid organic phase containing uranium and iron (for example a liquid organic phase comprising an organic extractant and an organic diluent) which does not have the drawbacks and disadvantages of the de-ironing method of the prior art mentioned above, notably in documents [1] and [2], and which resolves the problems of this method.

In particular, a need exists for such a method, which ensures selective separation of iron, while avoiding losses of uranium or phenomena of precipitation of impurities, which are bothersome for carrying out the method, observed during the implementation of methods of the prior art.

There further exists a need for a method for extracting uranium from an aqueous solution of an inorganic acid containing uranium and iron, comprising a step for de-ironing an organic phase derived from the treatment of said aqueous solution, which does not have the drawbacks of the methods for extracting uranium of the prior art, and which provides a solution to the problems posed by the methods for extracting uranium of the prior art.

There further exists a need for such a method which is simple and which thus has a limited number of unitary operations, which is reliable, robust and economic, which uses reagents that are notably easily and widely available, and which makes it possible to reduce costs.

DESCRIPTION OF THE INVENTION

This aim, and yet others, are attained, according to the invention, by a method for separating iron from an initial liquid organic phase containing uranium and iron, in which the initial liquid organic phase is placed in contact with an aqueous solution referred to as aqueous de-ironing (iron removal, deferrization) solution, whereby the iron passes into the aqueous solution to form a final liquid aqueous phase, and uranium remains in the initial liquid organic phase to form a final liquid organic phase referred to as de-ironed (iron-removed from which iron has been removed) organic phase; said method being characterised in that the aqueous de-ironing solution contains an inorganic acid and uranium, and does not contain iron.

The terms “does not contain iron” are generally taken to mean that the aqueous de-ironing solution contains from 0 to 10 ppm of iron, preferably contains 0 ppm of iron (is free of iron).

The method for separating iron according to the invention, also called de-ironing (iron removal method, deferrization method) fundamentally differs from the methods for separating iron of the prior art and notably from the method described in documents FR-A-2 596383 [1], and EP-A-239 501 [2], in that it uses a specific aqueous de-ironing solution which contains an inorganic acid and uranium and not uniquely an inorganic acid.

This specific aqueous de-ironing solution makes it possible, in a surprising manner, to eliminate, remove, selectively iron from the organic phase loaded with uranium and iron.

Indeed, when the organic phase is contacted with the aqueous de-ironing solution according to the invention a chemical displacement of iron by uranium to the aqueous phase takes place, which thereby ensures selective de-ironing (iron removal) of the organic phase loaded with uranium and iron.

In documents FR-A-2 596 383 [1] and EP-A-239 501 [2], during the de-ironing step, an acid is used that is selected from oxalic acid, a mixture of phosphoric and sulphuric acid or de-ironed sulphuric acid, without any addition of uranium, hence apart from the drawbacks mentioned above linked to the use of these acids, a selective elimination of iron due to the chemical displacement of iron by uranium can virtually not be obtained.

It has been shown (see examples), in a surprising manner, that the yield of de-ironing (iron removal, deferrization) an organic phase obtained with an aqueous solution of inorganic acid according to the invention loaded with uranium was much higher respectively than the yield of de-ironing (iron removal) obtained with an aqueous solution of inorganic acid not containing uranium.

The concept according to the invention of purification, de-ironing (iron removal) by chemical displacement applies to all sorts of organic phases, for example to all organic phases containing organophosphorous extractants.

The method of de-ironing (iron removal) according to the invention overcomes the drawbacks listed above due to the implementation, during the de-ironing (iron removal) step, of an acid selected from oxalic acid, mixtures of phosphoric and sulphuric acid, or de-ironed phosphoric acid. For example, in the method according to the invention, there is no loss of industrial phosphoric acid.

The method according to the invention only uses common inorganic reagents which are for example already present, among others, on phosphoric acid production sites, which can notably reduce the operating costs of the method.

The method according to the invention limits the number of unitary operations and eliminates disadvantageous impurities in an original manner by saturation of the solvent with beneficiable materials, namely uranium.

No limitation on the initial organic phase exists, and the method according to the invention may be applied with success to the treatment of any organic phase whatever the nature and the source.

The method according to the invention may notably apply to the treatment of an initial liquid organic phase which comprises an organic extraction system comprising an organic extractant or a mixture of organic extractant(s), diluted in an organic diluent non-reactive and non-miscible with water.

It has been shown (see Examples) that the de-ironing method according to the invention may be successfully implemented for the treatment of any organic phase of this type, whatever the nature of the organic extraction system.

In particular (see Examples), it has been shown that the de-ironing method according to the invention may be successfully implemented equally well with an organic extraction system comprising a single organic extractant as with an organic extraction system comprising a synergistic mixture of organic extractant(s), and whatever the nature of the extractant(s).

The organic extraction system may notably be selected from all the extraction systems described in documents [1] to [4] and documents FR-A-2442 796, FR-A-2 459 205, FR-A-2 494 258, and EP-A1-053 054, cited above, to the description of which reference is explicitly made in this respect and of which the passages relative to the extraction systems are consequently expressly included herein.

The organic extraction system may notably comprise an extractant selected from organophosphorous compounds and mixtures thereof.

Once again, the de-ironing method according to the invention may be successfully implemented with all these organophosphorous extractants, used alone or as a mixture.

Advantageously, the organic extraction system may comprise an extractant selected from acid organophosphorous compounds such as dialkylphosphoric acids, bifunctional organophosphorous compounds, neutral phosphine oxides such as trialkylphosphine oxides, and mixtures thereof.

In one embodiment, the extraction system may comprise the mixture of an acid organophosphorous compound and of a neutral phosphine oxide.

Advantageously, the acid organophosphorous compound may be selected from di(2-ethylhexyl) phosphoric acid (Di₂EHPA), bis(1,3-dibutoxy, 2-propyl) phosphoric acid (BIDIBOPP) and bis(1,3-dihexyloxy, 2-propyl) phosphoric acid (BIDIHOPP); and the neutral phosphine oxide is selected from trioctylphosphine oxide (TOPO), and di-n-hexyl octyl methoxy phosphine oxide (DinHMOPO).

In particular, the extractant system may be selected from the following mixtures of extractants:

-   -   a mixture of TOPO and Di₂EHPA;     -   a mixture of TOPO and BIDIBOPP;     -   a mixture of TOPO and BIDIHOPP;     -   a mixture of DinHMOPO and Di₂EHPA;     -   a mixture of DinHMOPO and BIDIBOPP;     -   a mixture of DinHMOPO and BIDIHOPP.

In another embodiment, the extraction system may comprise a mixture of a trialkylphosphoric acid and a trialkyl phosphate, such as TBP.

In yet another embodiment, the extraction system may comprise as extractant a compound which corresponds to the following general formula (I):

wherein: m is a whole number equal to 0, 1 or 2; R¹ and R², identical or different, are a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms;

R³ is:

-   -   a hydrogen atom;     -   a hydrocarbon group, saturated or unsaturated, linear or         branched, comprising from 1 to 12 carbon atoms and optionally         one or more heteroatoms;     -   a monocyclic hydrocarbon group, saturated or unsaturated,         comprising from 3 to 8 carbon atoms and optionally one or more         heteroatoms; or     -   a monocyclic aryl or heteroaryl group;         or instead R² and R³ together form a —(CH₂)_(n)— group wherein n         is a whole number ranging from 1 to 4;         R⁴ is a hydrocarbon group, saturated or unsaturated, linear or         branched, comprising from 2 to 8 carbon atoms, or a monocyclic         aromatic group; whereas R⁵ is a hydrogen atom or a hydrocarbon         group, saturated or unsaturated, linear or branched, comprising         from 1 to 12 carbon atoms.

Depending on the signification of R² and R³, the compound (extractant) of formula (I) may correspond:

-   -   either to the following specific formula (I-a):

wherein: m, R¹, R⁴ and R⁵ are such as defined previously; R² is a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms; whereas

R³ is:

-   -   a hydrogen atom;     -   a hydrocarbon group, saturated or unsaturated, linear or         branched, comprising from 1 to 12 carbon atoms and optionally         one or more heteroatoms;     -   a monocyclic hydrocarbon group, saturated or unsaturated,         comprising from 3 to 8 carbon atoms and optionally one or more         heteroatoms; or     -   a monocyclic aryl or heteroaryl group;         -   or to the following specific formula (I-b):

wherein m, n, R¹, R⁴ and R⁵ are such as defined previously.

According to the invention, “hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms”, is taken to mean any alkyl, alkenyl or alkynyl group, with linear or branched chain, which comprises 6, 7, 8, 9, 10, 11 or 12 carbon atoms.

In an analogous manner, “hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 2 to 8 carbon atoms”, is taken to mean any alkyl, alkenyl or alkynyl group, with linear or branched chain, which comprises 2, 3, 4, 5, 6, 7 or 8 carbon atoms.

Furthermore, “hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 1 to 12 carbon atoms and optionally one or more heteroatoms”, is taken to mean any group formed of a hydrocarbon chain, linear or branched, which comprises 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 carbon atoms, of which the chain may be saturated or, conversely, comprise one or more double or triple bonds and of which the chain may be interrupted by one or more heteroatoms or substituted by one or more heteroatoms or by one or more substituents comprising a heteroatom.

In this respect, it is specified that “heteroatom” is taken to mean any atom other than carbon or hydrogen, said atom being typically a nitrogen, oxygen or sulphur atom.

Furthermore, “monocyclic hydrocarbon group, saturated or unsaturated, comprising from 3 to 8 carbon atoms and optionally one or more heteroatoms” is taken to mean any cyclic hydrocarbon group that only comprises a single ring and of which the ring comprises 3, 4, 5, 6, 7 or 8 carbon atoms. This ring may be saturated or, conversely, comprise one or more double or triple bonds, and may comprise one or more heteroatoms or be substituted by one or more heteroatoms or by one or more substituents comprising a heteroatom, this or these heteroatoms being typically N, O or S. Thus, this group may notably be a cycloalkyl, cycloalkenyl or cycloalkynyl group (for example, a cyclopropane, cyclopentane, cyclohexane, cyclopropenyl, cyclopentenyl or cyclohexenyl group), a saturated heterocyclic group (for example, a tetrahydrofuryl, tetrahydrothiophenyl, pyrrolidinyl or piperidinyl group), an unsaturated but non-aromatic heterocyclic group (for example, pyrrolinyl or pyridinyl), an aromatic group or instead a heteroaromatic group.

In this respect, it is specified that “aromatic group” is taken to mean any group of which the ring meets the Hückel aromaticity rule and thus has a number of delocalised π electrons equal to 4n+2 (for example, a phenyl or benzyl group), whereas “heteroaromatic group” is taken to mean any aromatic group as has just been defined but in which the ring comprises one or more heteroatoms, this or these heteroatoms being typically selected from nitrogen, oxygen and sulphur atoms (for example, a furanyl, thiophenyl or pyrrolyl group).

Finally, the —(CH₂)_(n)— group in which n is a whole number ranging from 1 to 4 may be a methylene, ethylene, propylene or butylene group.

In the above specific formula (I-a), R¹ and R², which may be identical or different, are advantageously an alkyl group, linear or branched, comprising from 6 to 12 carbon atoms.

Even more, it is preferred that R¹ and R² are identical to each other and both are a branched alkyl group, comprising from 8 to 10 carbon atoms, the 2-ethylhexyl group being quite particularly preferred.

Furthermore, in the above specific formula (I-a):

-   -   m is, preferably, equal to 0;     -   R³ is advantageously a hydrogen atom, an alkyl group, linear or         branched, comprising from 1 to 12 carbon atoms, or a monocyclic         aryl group, preferably phenyl or ortho-, meta- or para-tolyl;         whereas     -   R⁵ is preferentially a hydrogen atom.

Even more, it is preferred that R³ is a hydrogen atom, a methyl, n-octyl or phenyl group.

Finally, in the above specific formula (I-a), R⁴ is, preferably, an alkyl group, linear or branched, comprising from 2 to 8 carbon atoms and, better still, from 2 to 4 carbon atoms such as an ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl or tert-butyl group, ethyl and n-butyl groups being quite particularly preferred.

Compounds of above specific formula (I-a) which have these characteristics are notably:

-   -   ethyl 1-(N,N-diethylhexylcarbamoyl)benzylphosphonate, which         corresponds to the above specific formula (I-a) in which m is         equal to 0, R¹ and R² are both a 2-ethyl-hexyl group, R³ is a         phenyl group, R⁴ is an ethyl group whereas R⁵ is a hydrogen         atom;     -   ethyl 1-(N,N-diethylhexylcarbamoyl)ethylphosphonate, which         corresponds to the above specific formula (I-a) in which m is         equal to 0, R¹ and R² are both a 2-ethyl-hexyl group, R³ is a         methyl group, R⁴ is an ethyl group whereas R⁵ is a hydrogen         atom;     -   ethyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, which         corresponds to the above specific formula (I-a) in which m is         equal to 0, R¹ and R² are both a 2-ethyl-hexyl group, R³ is an         n-octyl group, R⁴ is an ethyl group whereas R⁵ is a hydrogen         atom;     -   butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, which         corresponds to the above specific formula (I-a) in which m is         equal to 0, R¹ and R² are both a 2-ethyl-hexyl group, R³ is an         n-octyl group, R⁴ is an n-butyl group whereas R⁵ is a hydrogen         atom; and butyl 1-(N,N-dioctylcarbamoyl)nonylphosphonate, which         corresponds to the     -   above specific formula (I-a) in which m is equal to 0, R¹, R²         and R³ are all three an n-octyl group, R⁴ is an n-butyl group         whereas R⁵ is a hydrogen atom.

Among these compounds, ethyl 1-(N,N-diethyl-hexylcarbamoyl)nonylphosphonate and especially butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate (DEHCNPB) are quite particularly preferred.

In the specific family (I-b) above, R¹ is advantageously an alkyl group, linear or branched, comprising from 6 to 12 carbon atoms.

Furthermore, in this specific formula:

-   -   m is, preferably, equal to 0;     -   R⁴ is preferentially an alkyl group, linear or branched,         comprising from 2 to 8 carbon atoms and, better still, from 2 to         4 carbon atoms, whereas     -   R⁵ is, preferably, a hydrogen atom.

A compound of the above specific formula (I-b) which has these characteristics is notably ethyl (N-dodecylpyrrolidone)-1-phosphonate which corresponds to the specific formula (I-b) in which R¹ is a n-dodecyl group, R² and R³ form together an ethylene group (—CH₂—CH₂—), R⁴ is an ethyl group whereas R⁵ is a hydrogen atom.

The compounds of formula (I), (I-a), and (I-b), and the aforementioned specific compounds are described in document WO-A1-2013/167516 [5], to the description of which reference is expressly made in this respect.

In a particularly preferred manner, the extractant system is selected from Di₂EHPA, for example at a concentration of 0.5 M; a mixture of Di₂EHPA, preferably at a concentration of 0.5 M, and TOPO, preferably at a concentration of 0.125 M; a mixture of Di₂EHPA, preferably at a concentration of 0.2 M, and TBP, preferably at a concentration of 0.2 M; and butyl 1-(diethylhexyl carbamoyl) nonyl phosphonate (DEHCNPB) at a concentration of 0.1 M or 0.5 M.

It has been shown (see examples), that the method according to the invention may be successfully implemented with an organic phase resulting from the extraction of uranium by a first organic phase, or solvent phase, from an aqueous uranium bearing solution of mineral, inorganic acid, such as phosphoric acid, nitric acid or sulphuric acid. It has also been shown (see examples) that the method according to the invention could be successfully implemented whatever the origin of this aqueous uranium bearing solution of inorganic acid.

Thus, this aqueous uranium bearing solution of inorganic acid may equally well be an aqueous uranium bearing solution of phosphoric acid, such as industrial phosphoric acid, derived from the lixiviation, attack, of a natural phosphate ore, generally based on apatite, by sulphuric acid, as an aqueous uranium bearing solution of sulphuric acid or nitric acid derived from the lixiviation, attack, of a non-phosphate uranium bearing ore, for example non-apatite based, by respectively sulphuric acid or nitric acid.

Generally, the initial organic phase contains from 0.5 to 10 g/L of uranium; and from 0.1 to 10 g/L of iron.

Advantageously, the inorganic acid of the aqueous de-ironing (iron removal, deferrization) solution is selected from sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid, and mixtures thereof.

The preferred inorganic acid of the aqueous de-ironing solution is sulphuric acid.

Indeed, it is particularly interesting to use sulphuric acid which is widely available on phosphoric acid production sites because large amounts of sulphuric acid are consumed during the lixiviation of phosphate ores.

Advantageously, the concentration of inorganic acid of the aqueous de-ironing solution is from 0.1 M to 18 M, preferably from 1 to 1.5 M.

Advantageously, the quantity of uranium provided by the aqueous de-ironing solution is such that the concentration of uranium in the organic phase is at least equal to 50%, preferably at least equal to 60%, further preferably at least equal to 70% of the concentration of uranium corresponding to uranium saturation of the organic phase.

Advantageously, the concentration of uranium, expressed in [U], of the aqueous de-ironing solution is from 0.10 to 800 g/L, preferably from 30 to 50 g/L, for example 40 g/L.

The aqueous iron removal solution does not contain iron.

Generally, during the contacting (placing in contact), the initial organic phase is mixed with the aqueous de-ironing solution, then said mixture is decanted.

After the mixture has been decanted, are obtained, on the one hand, a final liquid aqueous phase containing the iron that was contained in the initial organic phase and, on the other hand, a final organic phase referred to as de-ironed (iron-removed, deferrized, from which iron has been removed) phase containing the uranium that was contained in the initial organic phase and not containing iron.

A definition of the phrase “not containing iron” when it relates this time to the final organic phase referred to as de-ironed (iron-removed, deferrized, from which iron has been removed) phase is given below.

Advantageously, the placing in contact is implemented in a battery of 1 to 5 mixers-decanters, for example 3 mixers-decanters, counter-current supplied with the initial organic phase and with the aqueous de-ironing solution.

Advantageously, the placing in contact may be carried out at a temperature of 0° C. to 70° C. within the limit of the flash point temperature of the organic diluent, preferably at a temperature from 40° C. to 45° C.

Advantageously, the O/A ratio of the flow rate of the initial organic phase to the flow rate of the aqueous de-ironing solution is from 1/5 to 5/1, for example 1/1.

Advantageously, the final aqueous phase contains more than 90% of the weight of iron contained in the initial organic phase, and less than 1% of the weight of uranium contained in the initial organic phase, and the de-ironed (iron removed, deferrized, from which iron has been removed) organic phase contains at least 90% of the weight of uranium contained in the initial organic phase, and less than 10% of the weight of iron contained in the initial organic phase.

Advantageously, the final organic phase referred to as de-ironed (iron removed, deferrized) organic phase contains less than 10 ppm of iron, preferably 0 ppm of iron (is free of iron).

Thanks to the method according to the invention, a selective elimination of iron is obtained which may be greater than 90%, in one contact. The Fe/U weight ratio in the de-ironed organic phase is generally less than 0.15%, which complies with the ASTM specifications.

The invention further relates to a method for extracting uranium, from a first aqueous solution of an inorganic acid containing uranium and iron, in which at least the following successive steps are carried out:

a) the first aqueous solution of inorganic acid is contacted with a first liquid organic phase; by means of which are obtained, on the one hand, a second liquid organic phase containing a majority by weight of the quantity of uranium contained in the first aqueous solution of inorganic acid and a minority by weight of the quantity of iron contained in the first aqueous solution of inorganic acid and, on the other hand, a second desuraniated (uranium removed, from which uranium has been removed) aqueous phase containing the inorganic acid, a minority by weight of the quantity of uranium contained in the aqueous solution of inorganic acid and a majority by weight of the quantity of iron contained in the aqueous solution of inorganic acid;

b) the iron is separated from the second liquid organic phase containing uranium and iron, by contacting the second liquid organic phase with a third aqueous solution referred to as aqueous de-ironing (iron removal, deferrization) solution, by means of which the iron passes into the aqueous de-ironing solution to form a final liquid aqueous phase, and uranium remains in the second liquid organic phase to form a final liquid organic phase referred to as de-ironed (deferrized, from which iron has been removed) organic phase;

said method being characterised in that the aqueous de-ironing solution contains an inorganic acid and uranium, and does not contain iron.

It should be recalled that the phrase “does not contain iron” is generally taken to mean that the aqueous de-ironing solution contains from 0 to 10 ppm of iron, preferably contains 0 ppm of iron (is free of iron).

In this method for extracting uranium and iron, from an aqueous solution of an inorganic acid, step b) is carried out by the de-ironing method according to the invention as has been described above, and all of the description of the de-ironing method provided above applies integrally to step b). The second liquid organic phase treated during step b) corresponds to the second liquid organic phase of step a) treated by the de-ironing method according to the invention. The third aqueous solution referred to as aqueous iron removal solution corresponds to the aqueous de-ironing solution used in the de-ironing method according to the invention and has been described in detail above.

It will further be understood that the first liquid organic phase differs from the second organic phase in that it does not contain either uranium, or iron and that it is thus exclusively constituted of organic compounds. For example, this first organic phase may be constituted by an organic extraction system comprising an organic extractant or a mixture of organic extractant(s) diluted in an organic diluent, non-reactive and non-miscible with water. Such an organic extraction system has already been described in detail above.

In step a), the second organic phase obtained contains at least 90% by weight, for example from 95 to 100% by weight, of the quantity of uranium contained in the first aqueous solution of inorganic acid (starting solution), and from 0.1 to 50% by weight of the quantity of iron contained in the first aqueous solution of inorganic acid; and the second desuraniated (from which uranium has been removed) aqueous phase obtained contains the inorganic acid, from 0 to 10% by weight of the quantity of uranium, and from 50 to 99.9%, for example from 80 to 90% by weight of the quantity of iron contained in the first aqueous solution of inorganic acid (starting solution).

Generally, the second organic phase obtained at the end of step a) contains from 0.5 to 10 g/L of uranium, and from 0.1 to 10 g/L of iron, and the second aqueous phase obtained at the end of step a) contains from 0 to 100 mg/L of uranium, and from 0.1 to 6 g/L of iron.

The method according to the invention differs fundamentally from methods of the prior art in that the step of de-ironing b) is carried out with a specific aqueous de-ironing solution which contains an inorganic acid and uranium, and which does not contain iron.

In other words, the step of de-ironing b) is carried out by implementing the de-ironing method according to the invention as has been described above, and thus has all the advantages inherent in this de-ironing method.

This aqueous de-ironing solution makes it possible, in a surprising manner, to eliminate selectively iron from the second organic phase, or solvent phase loaded with uranium and with iron.

Indeed, when the organic phase or solvent phase obtained in step a) in placed in contact with the aqueous de-ironing solution according to the invention a chemical displacement of iron by uranium to the aqueous phase takes place, which thereby ensures selective iron removal from the organic phase, loaded with uranium and with iron.

The implementation of the aqueous solution according to the invention in the iron removal step b), which thereby makes it possible to obtain selective elimination of iron, has never been described in the prior art.

Thus, in documents FR-A-2 596 383 [1] and EP-A-239 501 [2], during the step of iron removal an acid is used that is selected from oxalic acid, a mixture of phosphoric and sulphuric acid or de-ironed sulphuric acid, without any addition of uranium, hence, apart from the aforementioned drawbacks linked to the use of these acids, selective elimination of iron due to the chemical displacement of iron by uranium cannot be obtained.

The method according to the invention, notably on account of the implementation of the aforementioned specific solution during the de-ironing (iron removal) step, does not have the drawbacks, defects, limitations and disadvantages of methods of the prior art and solves the problems of methods of the prior art.

In particular, for example, in the method according to the invention, there is no loss of industrial phosphoric acid.

The method according to the invention notably during the de-ironing step, only uses inorganic reagents that are already present on phosphoric acid production sites.

For example, it is particularly interesting to use sulphuric acid which is widely available on phosphoric acid production sites because large amounts of sulphuric acid are consumed during the lixiviation of phosphate ores.

To summarise, the method according to the invention, while being more economical, enables among others a selective elimination of iron by chemical displacement while avoiding losses of uranium and phenomena of precipitation of iron which hinder the carrying out of the method. The method according to the invention limits the number of unitary operations and eliminates disadvantageous impurities by saturation of the solvent with the beneficiable materials, namely uranium.

Advantageously, the inorganic acid of the first aqueous solution of inorganic acid of step a) is a solution of phosphoric acid, sulphuric acid or nitric acid.

Advantageously, the first aqueous solution of inorganic acid of step a) contains from 0.1 to 10 g/L of iron, and from 0.05 to 10 g/L of uranium.

It has been shown (see Examples), that the method according to the invention may be implemented with success with a large variety of first aqueous solutions of inorganic acid whatever their origin, namely for example an aqueous uranium bearing solution of phosphoric acid, such as industrial phosphoric acid, coming from the lixiviation, attack, of a natural phosphate ore, generally based on apatite, by sulphuric acid, or an aqueous uranium bearing solution of sulphuric acid or nitric acid, coming from the lixiviation, attack, of a non-phosphate uranium bearing ore, for example non-apatite based, respectively by sulphuric acid or nitric acid as described previously.

Advantageously, step a) is carried out at a temperature from 30° C. to 35° C., in a battery of 5 mixers-decanters counter-current supplied with organic phase and with aqueous phase, and with an O/A ratio of the flow rate of organic phase to the flow rate of aqueous phase of 1/6 to 1/8, for example 1/7.

The method according to the invention may further comprise a step c) wherein the de-ironed (iron removed) organic phase obtained in step b) is placed in contact with an aqueous solution of a complexing base; by means of which are obtained, on the one hand, an aqueous phase loaded with uranium and, on the other hand, an organic phase free of uranium, and further containing the complexing base.

Advantageously, the complexing base is a carbonate of an alkali or alkaline-earth metal such as sodium carbonate.

The method according to the invention may further comprise a step d) wherein the organic phase free of uranium, further containing the complexing base obtained in step c), is placed in contact with the aqueous phase coming from step b) and neutralised, whereby are obtained, on the one hand, an organic phase consisting of the organic solvent which is sent back to step a) and, on the other hand, an aqueous phase.

The method according to the invention may further comprise a step e) in which the aqueous phase loaded with uranium obtained in step c), is contacted with a base such as sodium hydroxide, whereby a uranate precipitate, such as a sodium urinate precipitate, which is separated, and an aqueous solution which is sent to step c) after addition of a complexing base, are obtained.

Advantageously, all or part of the uranate precipitate, such as sodium uranate, obtained in step e), is dissolved in an inorganic acid such as sulphuric acid, and the aqueous solution obtained containing an inorganic acid and uranium is sent to step b) after having optionally adjusted the concentration of inorganic acid.

The invention will be better understood on reading the detailed description that follows of an embodiment of the method according to the invention for extracting uranium from an aqueous solution of mineral acid, containing uranium, iron and optionally one or more other impurities.

This description is given for illustrative purposes and is non-limiting and is made with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the method according to the invention.

It should be noted that all the indications given in FIG. 1 without exception, for example concerning the reagents used, the concentrations, the temperatures, etc., are only given as examples and do not in any way constitute a limitation.

FIG. 2 is a graph which shows the kinetic profiles of the yield of selective de-ironing (iron removal) of the solvent for different initial concentrations of uranium: namely 0 g/L (curve A), 10 g/L (curve B), 20 g/L (curve C), 30 g/L (curve D), 35 g/L (curve E), 40 g/L (curve F), 50 g/L (curve G), 60 g/L (curve H), 70 g/L (curve I), 100 g/L (curve J), in the aqueous phase during the complementary tests of example 2.

On the Y-axis is given the de-ironing (iron removal) yield of the solvent (in %) and on the X-axis is given the time (in min.).

FIG. 3 is a graph which gives the de-ironing yield (or removal of Fe in %) from the loaded solvents A, B, C, D, E, F, G, H, L, I, J, K, and M prepared in example 4, in one contact, either with pure 1.5 M sulphuric acid (for each test A, B, C, D, E, F, G, H, L, I, J, K, and M: left bar), or with 1.5 M sulphuric acid containing uranium (for each test: right bar).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The detailed description that is made relates to one embodiment of the method according to the invention, for extracting uranium from an aqueous solution of mineral acid, containing uranium and iron, in which the aqueous solution of mineral acid is an aqueous solution of phosphoric acid containing uranium and iron.

It is clearly obvious that, subject to some minor adaptations within the reach of the man skilled in the art, the method described hereafter may easily be implemented with aqueous solutions of other mineral acids such as sulphuric acid or nitric acid, containing uranium and iron.

The aqueous solution of phosphoric acid containing uranium and iron which is treated by the method according to the invention, referred to as “starting” aqueous solution of phosphoric acid (1), generally has a concentration expressed in P₂O₅ from 26% to 32% by weight, preferably from 28% to 32% by weight, for example from 28% to 30% by weight expressed in P₂O₅.

The aqueous solution of phosphoric acid treated by the method according to the invention generally contains from 0.05 to 1 g/L of uranium, notably from 0.08 to 0.4 g/L of uranium (expressed in [U]).

The uranium in this aqueous solution is generally in solution in the form of U(VI) and U(IV), the latter having to be the subject of a prior step of oxidation into U(VI).

The aqueous solution of phosphoric acid treated by the method according to the invention generally contains from 0.1 to 10 g/L of iron, notably from 1 to 6 g/L of iron.

This aqueous solution is generally an aqueous solution, known as attack solution, obtained during the attack of phosphate ores by sulphuric acid.

Before being treated by the method according to the invention, the phosphoric acid solution containing uranium and iron may undergo one or more pre-treatment steps (2), notably a step of flash cooling, then a Solid/Liquid separation step, then a step of oxidation for example by hydrogen peroxide.

The cooling step makes it possible for example to cool the attack solution, which is hot.

The Solid/Liquid separation step makes it possible to separate the gypsum in super saturation in the solution.

The oxidation step, for example by hydrogen peroxide or by another oxidant such as NaClO₃, makes it possible to oxidise uranium in the form of U (IV) into uranium in the form of U (VI).

According to the invention, in the first step or step a) also called extraction step (3), of the method according to the invention, the aqueous solution of phosphoric acid (1) is placed in contact with an organic extraction solvent (4) comprising a single extractant or instead a synergistic mixture of extractants, diluted in an organic diluent, non-reactive and non-miscible with water.

Synergistic mixture of extractants is taken to mean that this mixture has extractive properties higher than or even much higher than the extractive properties obtained by the simple addition of the extractive properties of each of the extractants which constitute the mixture of extractants.

Examples of such extractants, and of such synergistic mixtures of extractants, are known to the man skilled in the art in this technical field and are given for example in documents [1] to [4] and documents FR-A-2 442 796, FR-A-2 459 205, FR-A-2 494 258, and EP-A1-053 054 cited above, to the description of which reference may be made in this respect.

Preferred extractants used alone are Di₂EHPA, preferably at a concentration of 0.5 M.

Other preferred extractants used alone are the bifunctional extractants of document [4] described above, such as DEHCNPB, preferably at a concentration of 0.1 M to 0.5 M.

Synergistic mixtures of extractants may consist for example of a neutral phosphine oxide and of an acid-organophosphorous compound, in particular a mixture of a dialkylphosphoric acid and of a trialkylphosphine oxide.

Preferably, the acid-organophosphorous compound of the mixture, such as a dialkylphosphoric acid, is selected from bis 2-ethylhexyl phosphoric acid (Di₂EHPA), bis dibutoxy 1,3 propyl 2 phosphoric acid (BIDIBOPP) and bis dihexyloxy 1,3 propyl 2 phosphoric acid (BIDIHOPP); and the neutral phosphine oxide is selected from trioctylphosphine oxide (TOPO) and di-n-hexyl octyl methoxy phosphine oxide (DinHMOPO).

Preferred extractant mixtures of this type are the following:

-   -   TOPO/Di₂EHPA;     -   TOPO/BIDIBOPP;     -   TOPO/BIDIHOPP;     -   DinHMOPO/Di₂EHPA;     -   DinHMOPO/BIDIBOPP;     -   DinHMOPO/BIDIHOPP.

A particularly preferred synergistic extractant mixture is a mixture of D₂EHPA and TOPO, preferably a mixture of 0.5 M D₂EHPA and 0.125 M TOPO.

Another synergistic mixture of extractants is a mixture of D₂EHPA and TBP, preferably a mixture of 0.2 M D₂EHPA and 0.2 M TBP: this is the mixture used in the “DAPEX” method.

The organic diluent, non-miscible and non-reactive with water, is generally selected from liquid hydrocarbons.

These liquid hydrocarbons may be selected from aromatic hydrocarbons such as benzene, aliphatic hydrocarbons such as n-heptane and n-octane, and mixtures thereof. A mixture of hydrocarbons that is suitable as diluent according to the invention is kerosene.

Aliphatic kerosenes, such as the products available under the denomination ShellSol®, may thus be suitable for use in the organic diluent.

Other mixtures of hydrocarbons that are suitable as diluent according to the invention are the products available under the denomination SANE® such as SANE® IP 185.

This extraction step may be carried out in static mode or in dynamic mode.

This extraction step may be carried out in any suitable extraction apparatus, for example in one or more mixers-decanters, and/or in one or more agitated or pulsed columns.

The man skilled in the art may easily determine the number of suitable theoretical stages that the extraction apparatus has to comprise to carry out the extraction.

Generally, this extraction step is carried out with a battery of mixers-decanters in dynamic counter-current operation, that is to say with the organic phase and the aqueous phase circulating in counter-current to each other from the first, respectively the last, of the mixers-decanters, up to the final respectively first, of the mixers-decanters.

The number of mixers-decanters may range from 1 to 10, notably from 1 to 5.

Preferably, 5 mixers-decanters, in other words 5 mixing-decanting stages are implemented.

The supply with organic phase may then take place for example in stage 1, whereas the supply with aqueous phase takes place for example in stage 5.

The overall O/A ratio for all of the battery of mixers-decanters, all of the stages, is generally 1/6 i.e. 0.1667, to 1/8 i.e. 0.1250, depending on the initial concentration of uranium in the starting solution of phosphoric acid.

It should be recalled that O/A designates the ratio of the flow rate of organic phase to the flow rate of aqueous phase.

This step of the method is generally carried out at a temperature from 10° C. to 60° C., notably 10° C. to 50° C. It may be carried out at room temperature, for example 20° C. to 25° C., but it is preferably carried out at a temperature from 30° C. to 35° C., which makes it possible to obtain a relatively rapid kinetic for extracting uranium.

The mixing time per mixer is generally from 0.5 to 5 minutes, preferably 2 minutes, when the mixing is carried out within the preferred range of temperatures indicated above.

The dwell time in the decanters, per stage, is generally from 2 to 10 minutes, preferably 5 minutes, when the mixing is carried out within the preferred range of temperatures indicated above.

The yield for extracting uranium during this step is generally greater than or equal to 95%, preferably greater than or equal to 97%, further preferably greater than or equal to 98%.

Uranium leakage is generally less than or equal to 10 mg/L, preferably less than or equal to 5 mg/L, further preferably less than or equal to 3 mg/L.

At the end of this extraction step (3), are obtained, on the one hand, an organic phase (5) which contains from 90 to 100% by weight, for example 95% by weight, of the quantity of uranium contained in the aqueous solution of phosphoric acid (starting solution), and from 0.1 to 10% by weight of iron contained in the aqueous solution of phosphoric acid; and, on the other hand, a desuraniated (uranium-removed) aqueous phase (6) which contains phosphoric acid, from 0 to 10% by weight of the uranium, and from 80% to 99.9% by weight of the iron contained in the aqueous solution of phosphoric acid (starting solution).

The organic phase (5) obtained at the end of step a) of extraction (3) thus generally contains from 0.5 to 10 g/L of uranium and from 0.1 to 10 g/L of iron whereas the desuraniated (uranium-removed) aqueous phase (6) obtained at the end of step a) thus generally contains from 0 to 100 mg/L of uranium and from 0.1 to 6 g/L of iron.

The uranium-removed aqueous phase (6) may be optionally subjected to one or more post-treatments (7) selected for example from a coalescence treatment and a treatment with activated carbon in order notably to eliminate organic matters (coming from scavenging of the organic phase in the aqueous phase), and the phosphoric acid thereby recovered, which has a concentration expressed in P₂O₅ from 26% to 32% by weight, preferably from 28% to 32% by weight, for example from 28% to 30% by weight, analogous to the starting phosphoric acid, may next be used for example in fertiliser production plants.

The organic phase (5) obtained at the end of step a) (3), or extraction step, generally has a high Fe/U ratio of the order of 0.1 to 1, notably 0.5.

Next, during a step b), or de-ironing step (step of iron removal) of the solvent (8) according to the invention, the organic phase (5) obtained in step a) is placed in contact with an aqueous de-ironing solution (9).

According to the invention, the aqueous de-ironing solution (9) contains an inorganic acid and uranium, and does not contain iron.

The inorganic acid of the aqueous de-ironing solution (9) may be selected from sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid, and mixtures thereof.

The preferred inorganic acid of the aqueous de-ironing solution is sulphuric acid.

Advantageously, the concentration of inorganic acid of the aqueous de-ironing solution such as sulphuric acid is from 1 to 1.5 M.

The concentration of uranium of the aqueous de-ironing solution is preferably from 35 to 40 g/L, for example 40 g/L.

Indeed, at constant acidity the initial concentration of uranium constitutes a key parameter on the yields of selective de-ironing of the solvent.

The technical-economic optimum seems to be comprised in the aforementioned range of 35 to 40 g/L of uranium initially contained in the aqueous influent for an elimination of iron of the order of 90% in one contact.

Exhaustion (depletion) tests of the aqueous phase have also shown the absence of salting out of iron on the solvent and efficient uranium exhaustion generally in three successive contacts with contact times less than 5 minutes.

It has also been shown that the concept of selective de-ironing of the solvent by chemical displacement was valid whatever the acid, i.e. sulphuric, nitric, hydrochloric, or phosphoric.

This step of the method is generally carried out at a temperature from 10° C. to 50° C.

It may be carried out at room temperature, for example 20° C. to 25° C., but it is preferably carried out at a temperature from 40° C. to 45° C.

Indeed, the experimental results show a notable influence of the temperature on this step with a substantial gain in the mixing time necessary to attain the required performance.

As an example, the suitable contact time at 40° C. seems to be comprised between 5 and 10 minutes instead of the 30 minutes necessary at 20° C.

This step of de-ironing (iron removal step) (8) may be implemented in any suitable contacting apparatus, and be carried out in static mode or in dynamic mode.

Generally this step de-ironing (iron removal step) (8) is carried out with a battery of mixers-decanters in dynamic counter-current operation.

The number of mixers-decanters may range from 1 to 5.

Preferably, 3 mixers-decanters, in other words 3 mixing-decanting stages are implemented.

The supply with organic phase (5) takes place in stage 1, whereas the supply with aqueous phase (9) takes place in stage 3 which is also called “super-stage”.

The overall O/A ratio for all of the battery of mixers-decanters, all of the stages, is generally from 1/5 to 5/1.

A preferred overall O/A ratio is 1/1.

The contact time is generally of the order of 10 minutes for stage 3, that is to say for the aqueous supply stage, and 3 minutes for the two other stages.

The dwell time in the decanter is generally 5 minutes at the most.

In step b) (8) are obtained, on the one hand, an aqueous phase (10) containing from 50% to 90% of the iron contained in the organic phase (5) obtained in step a) and, on the other hand, a de-ironed (iron removed) organic phase (11) containing at least 85% by weight of the uranium contained in the organic phase (5) obtained in step a) and not containing iron, free of iron. “De-ironed”, “Iron-removed”, “from which iron has been removed, “free of iron”, “not containing iron” are generally taken to mean that this organic phase (11) contains less than 10 mg/L of iron, for example 5 mg/L of iron, or even 0 mg/L of iron.

The organic phase (11) obtained at the end of the de-ironing step (iron removal step) b) (8) thus generally contains from 0.5 to 60 g/L of uranium and from 0 to 10 mg/L of iron, whereas the desuraniated (uranium-removed) aqueous phase (10) obtained at the end of step b) thus contains generally from 0 to 1 g/L of uranium and from 0 to 2 g/L of iron. This aqueous phase (10) is an acid phase containing the inorganic acid described above.

The method according to the invention further generally comprises a step c), also called back extraction of uranium (12), in which the organic phase, de-ironed and loaded with uranium (11), obtained at the end of step b) of de-ironing of the solvent is contacted with an aqueous solution of a complexing base (13).

The complexing base may be selected from alkali metal carbonates, such as sodium carbonate, alkaline-earth metal carbonates, and ammonium carbonates.

The concentration of complexing base such as sodium carbonate of the aqueous solution is generally from 1 to 2 M, for example 1.5 M.

This step of back extraction (12) may be carried out in static mode or in dynamic mode. It may be carried out in any suitable extraction apparatus.

This step of back extraction (12) is generally carried out with a battery of mixers-decanters in dynamic counter-current operation.

The number of mixers-decanters may range from 1 to 5.

Preferably, 3 mixers-decanters, in other words 3 mixing-decanting stages are implemented.

The supply with organic phase (11) may take place, for example, in stage 1, whereas the supply with aqueous phase (13) may take place, for example, in stage 3.

The overall O/A ratio for all of the battery of mixers-decanters, all of the stages, is generally from 1/2 to 2/1, depending on the initial concentration of uranium in the starting organic phase.

A preferred overall O/A ratio is 1/1.

This step of back extraction of uranium (12) of the method is generally carried out at a temperature from 10° C. to 50° C.

It may be carried out at room temperature, for example 20° C. to 25° C., and a satisfactory separation of phases is obtained even at 25° C., but an increase in the operating temperature makes it possible to improve the performance.

The step of back extraction of uranium (12) is thereby carried out preferably at a temperature from 40° C. to 45° C., which makes it possible to obtain a relatively rapid kinetic of back extraction of uranium.

The mixing time is generally from 1 to 10 minutes, preferably 5 minutes, when the mixing is carried out within the preferred range of temperatures indicated above.

At the end of this step of back extraction of uranium (12) are obtained, on the one hand, an aqueous phase loaded with uranium (14) and, on the other hand, an organic phase (15) constituted by the organic solvent, free of uranium.

The aqueous phase loaded with uranium (14) generally contains from 5 to 80 g/L of uranium and from 0 to 100 mg/L of iron and the organic phase free of uranium (15) generally contains from 0 to 100 mg/L of uranium and from 0 to 10 mg/L of iron.

The method according to the invention further generally comprises a step d), referred to as step of acidification of the solvent (16), in which the organic phase free of uranium (15), further containing the complexing base coming from step c) (12)—in other words the desuraniated (uranium-removed) solvent coming from the step of back extraction of uranium (12)—is contacted with the aqueous phase (10) coming from step b), that is to say the step of de-ironing of the solvent (8). The acid concentration could optionally be adjusted if necessary.

This step of acidification of the solvent (16) may be carried out in static mode or in dynamic mode.

This step of acidification (16) may be carried out in any suitable contacting apparatus.

This step of acidification (16) is generally carried out with one mixer-decanter or a battery of mixers-decanters, for example from 1 to 8 mixers-decanters in dynamic counter-current operation.

Preferably, a single mixer-decanter, in other words a single mixing-decanting stage is implemented.

The overall O/A ratio for the single mixer-decanter, or for all of the battery of mixers-decanters, all of the stages, is generally from 1/5 to 5/1.

A preferred overall O/A ratio is 1/1.

This step of acidification (16) of the method according to the invention is generally carried out at a temperature from 10° C. to 50° C.

It may be carried out at room temperature, for example 20° C. to 25° C., but an increase in the operating temperature makes it possible to improve the performance.

The step of acidification (16) is thereby carried out preferably at a temperature from 40° C. to 45° C.

The mixing time is generally from 1 to 10 minutes per stage, preferably 5 minutes, when the mixing is carried out within the preferred range of temperatures indicated above.

At the end of this step of acidification of the solvent (16), are obtained, on the one hand, an organic phase (4) constituted by the organic solvent regenerated in acid form which is sent to step a) of extraction (3) and, on the other hand, an aqueous phase (17).

This aqueous phase (17) contains iron, for example at a level from 0 to 2 g/L, and the inorganic acid that was contained in the aqueous de-ironing solution implemented during the step of iron removal b) at a concentration of 1 to 1.5 M.

This aqueous phase (17) may be beneficiated. Thus, if the inorganic acid is sulphuric acid, this aqueous phase (17) may be recycled to a step of phosphate ore lixiviation (18).

The aqueous phase loaded with uranium (14) obtained at the end of the step of back extraction of uranium is generally treated in a step e), referred to as step of precipitation of uranate (19), in the course of which this aqueous phase loaded with uranium (14) is contacted with a base (20) such as sodium hydroxide whereby a uranate precipitate such as a sodium uranate precipitate is obtained, which is separated, and an aqueous solution free of uranium (21) is obtained which is sent back to step c) of back extraction of uranium (12) after a complexing base such as sodium carbonate has been added thereto.

The uranium contained in the aqueous phase loaded with uranium obtained at the end of the step of back extraction of uranium (12) may be in various forms.

If the complexing base is an alkali or alkaline-earth metal carbonate, such as sodium carbonate, then the uranium is in the form of uranyl tricarbonate of alkali or alkaline-earth metal, such as sodium uranyl tricarbonate.

The uranium is thus made to precipitate by addition of a base (20) such as sodium hydroxide, to the aqueous phase (14), for example at a temperature of 80° C. for a duration of 1 hour.

An uranate precipitate is thereby obtained, for example a sodium diuranate precipitate (SDU) or sodium uranate precipitate, if sodium hydroxide was used for the precipitation.

This uranate precipitate is separated by any suitable solid-liquid separation method, for example by filtration.

All or part (22) of this uranate precipitate, such as sodium uranate obtained in the step e) of precipitation (19), may be dissolved, during a step referred to as re-dissolution of the uranate (23), in an inorganic acid (24) such as sulphuric acid, at a pH for example of 3 to 3.5.

The inorganic acid (24) used for the dissolution may be selected from the same acids as those already mentioned for the aqueous de-ironing (iron removal) solution, namely, sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid, and mixtures thereof.

The preferred inorganic acid is sulphuric acid.

Following the dissolution of the uranate, a dissolution aqueous solution is thereby obtained containing an inorganic acid such as sulphuric acid, and uranium in the form of uranyl sulphate.

All or part of this dissolution aqueous solution (25) may be sent to step b) (8) to serve as aqueous de-ironing solution (9) after an optional adjustment of the acid concentration to obtain the desired acid concentration for the uranium bearing acid aqueous de-ironing solution (9).

Thus, an input of inorganic acid (26), such as sulphuric acid, could be made on the pipe carrying the dissolution aqueous solution (25).

As has been specified above, the concentration of inorganic acid of the aqueous de-ironing solution (9) such as sulphuric acid is in fact advantageously from 1 to 1.5 M, and the concentration of uranium of the aqueous de-ironing solution is advantageously from 35 to 40 g/L, for example 40 g/L.

All or part (27) of the uranate precipitate may be placed in vessels such as drums during a step referred to as “uranate drumming” (28).

All or part (29) of the dissolution aqueous solution of the uranate may optionally be sent to an optional step of precipitation (30) by hydrogen peroxide (31), generally carried out at room temperature, at the end of which a precipitate of uranium peroxide UO₄ (32) is obtained, which may be optionally separated by any suitable solid-liquid separation method, for example by filtration.

The precipitate of uranium peroxide may then be placed in vessels such as drums during a step referred to as “UO₄ drumming” (33).

In total, the method according to the invention implements for example twelve mixing-decanting stages.

The invention will now be described with reference to the following examples, given for indicative purposes and non-limiting.

EXAMPLES Example 1

In this example, the influence is shown of the initial concentration of uranium in the de-ironing solution used in the method according to the invention for separating iron and from a liquid organic phase.

In order to study the influence of the initial concentration of uranium in the aqueous de-ironing solution, laboratory-scale tests were carried out in separating funnels in the following conditions:

-   -   Aqueous iron removal solution (Aqueous phase A): 3 M H₂SO₄         containing uranium at a variable concentration;     -   Initial organic phase (Organic phase O), solvent loaded with         uranium and with iron: Synergistic mixture of 0.5 M D₂EHPA and         0.125 M TOPO in the diluent Isane® IP185, loaded with uranium         [U]=1200 mg/L and with iron [Fe]=526 mg/L;     -   Ratio of O/A phases=1/1.     -   Room temperature (22° C.).     -   Variable contact time.

The kinetics of back extraction of uranium and iron from the solvent are determined by analytical monitoring of the concentrations in the aqueous phase after contact with the solvent.

The results of these tests are presented in Table I below.

TABLE I Kinetic monitoring of the concentrations of uranium and iron after contact of the loaded solvent with a 3M H₂S0₄ solution containing uranium at a variable concentration (O/A = 1/1, room temperature). Aqueous phase Organic phase U_(initial) Time U Fe U Fe Solvent iron (g/L) (min) (mg/L) (mg/L) (mg/L) (mg/L) removal 0 0 0 0 1,200 526 0.0% 5 1 27 1,199 499 5.2% 10 1 77 1,199 450 14.6% 15 1 108 1,199 418 20.6% 30 1 183 1,199 343 34.7% 45 1 200 1,199 326 38.1% 60 1 214 1,199 312 40.8% 50 0 4,7357 0 1,200 526 0.0% 5 6,138 140 42,420 386 26.7% 10 5,970 318 42,588 208 60.4% 15 5,681 400 42,877 126 76.1% 30 5,632 482 42,926 44 91.6% 45 5,517 480 43,041 46 91.2% 60 5,652 489 42,905 37 93.0% 60 0 57,387 0 1,200 526 0.0% 5 13,936 128 44,652 398 24.4% 10 13,303 307 45,285 219 58.4% 15 13,177 416 45,411 110 79.1% 30 12,914 472 45,673 54 89.7% 45 12,763 492 45,824 34 93.6% 60 13,074 491 45,513 35 93.4% 70 0 66,626 0 1,200 526 0.0% 5 20,713 142 47,113 385 26.9% 10 20,512 331 47,315 195 62.8% 15 20,137 411 47,689 115 78.2% 30 20,074 475 47,753 52 90.2% 45 19,958 478 47,868 48 90.9% 60 20,230 478 47,596 48 90.9% 100 0 95,757 0 1,200 526 0.0% 5 43,987 125 52,971 401 23.8% 10 43,346 330 53,611 197 62.6% 15 43,049 388 53,908 138 73.8% 30 42,722 457 54,235 69 86.9% 45 42,819 459 54,139 67 87.2% 60 43,330 457 53,627 69 86.9%

These experimental results show a marked increase in the yield of iron removal from the moment that the aqueous phase initially contains, in accordance with the method according to the invention, uranium in high concentrations.

Furthermore, the kinetic of elimination of iron seems to be of the order of 30 minutes, the time for which the yield of de-ironing of the solvent reaches a plateau above 90% in one contact, except for a pure sulphuric solution for which the kinetic seems to be slower.

Example 2

In order to complete the preceding data obtained in example 1, complementary, further laboratory-scale tests were carried out in separating funnels in the following conditions:

-   -   Aqueous de-ironing solution (Aqueous phase A): 3 M H₂SO₄         containing uranium at a variable concentration;     -   Initial organic phase (Organic phase O), solvent loaded with         uranium and with iron: Synergistic mixture of 0.5 M Di₂EHPA and         0.125 M TOPO in the diluent Isane® IP185, loaded with uranium         [U]=1093 mg/L and with iron [Fe]=437 mg/L (The composition of         the solvent is thus slightly different from the composition of         the solvent in the tests of example 1 where[U]=1.2 g/L and         [Fe]=526 mg/L);     -   Ratio of O/A phases=1/1;     -   Room temperature (22° C.).

The kinetics of back extraction of uranium and iron from the solvent are determined by analytical monitoring of the concentrations in the aqueous phase after contact with the solvent.

The results of these complementary, further tests are presented in Table II below.

TABLE II Kinetic monitoring of the concentrations of uranium and iron after contact of the loaded solvent with a 3M H₂S0₄ solution containing uranium at a variable concentration (O/A = 1/1, room temperature) during the complementary tests. Aqueous phase Organic phase U_(initial) Time U Fe U Fe Solvent iron (g/L) (min) (mg/L) (mg/L) (mg/L) (mg/L) removal 10 0 9,965 0 1,093 437 0.0% 5 21 58 11,037 379 13.3% 15 21 126 11,037 311 28.9% 30 19 171 11,039 266 39.2% 120 21 249 11,037 188 57.0% 20 0 21,208 0 1,093 437 0.0% 5 103 87 22,198 351 19.8% 15 64 117 22,237 320 26.8% 30 69 245 22,233 192 56.1% 120 72 282 22,229 155 64.5% 30 0 31,912 0 1,093 437 0.0% 5 483 124 32,522 314 28.3% 15 350 259 32,654 178 59.2% 30 323 322 32,682 115 73.6% 120 345 368 32,660 69 84.2% 35 0 36,698 0 1,093 437 0.0% 120 1,284 375 36,507 63 85.6% 40 0 41,485 0 1,093 437 0.0% 5 2,842 180 39,736 257 41.2% 15 2,768 313 39,810 125 71.5% 30 2,553 383 40,025 54 87.6% 120 2,541 390 40,037 47 89.2%

The complementary tests of this example reveal a notable influence of the initial concentration of uranium in the de-ironing solution, notably for the low concentrations selected. Thus, the yield of iron removal (de-ironing yield) increases with the concentration of uranium while reducing the kinetic of iron removal.

Furthermore, FIG. 2 shows the kinetic profiles of the yield of selective iron removal (de-ironing) of the solvent for different initial concentration of uranium (namely 0 g/L, 10 g/L, 20 g/L, 30 g/L, 35 g/L, 40 g/L, 50 g/L, 60 g/L, 70 g/L, 100 g/L) in the aqueous phase during the complementary tests of example 2.

This figure shows, moreover, two distinct populations:

-   -   when the concentration of uranium is comprised between 0 and 30         g/L, for which the yield of iron removal increases and the time         to reach kinetic equilibrium decreases;     -   when the concentration of uranium is comprised between 35 and         100 g/L, for which the curves are superimposed, not just from         the thermodynamic viewpoint (de-ironing yield, yield of iron         removal) but also from the kinetic viewpoint (plateau reached at         the end of 30 minutes at room temperature);     -   finally, the kinetic for extracting uranium seems for its part         relatively rapid with an optimum comprised between 5 and 10         minutes at room temperature.

Example 3

In this example, tests are carried out in which the method according to the invention for separating iron from a liquid organic phase is implemented in a battery of 3 mixers-decanters (MD) in dynamic counter-current operation.

The conditions for these tests are the following:

-   -   Aqueous de-ironing solution (Aqueous phase A): 3 M H₂SO₄         ([H+]=5.9 mol/L) containing 39.76 g/L of uranium, with a supply         flow rate of 120 mL/h;     -   Initial organic phase (Organic phase O), solvent loaded with         uranium and iron: Synergistic mixture of 0.5 M D₂EHPA and 0.125         M TOPO in the diluent Isane® IP185, loaded with uranium [U]=994         mg/L and with iron [Fe]=307 mg/L, with a supply flow rate of 120         mL/h;     -   Useful volume: 50 ml mixer and 200 ml decanter for stage 3;     -   Useful volume: 30 ml mixer and 200 ml decanter for stages 1 and         2;     -   Supply with organic phase in stage 1;     -   Supply with aqueous phase in stage 3;     -   Ratio of O/A phases=1/1;     -   Temperature: 40° C. with double-walled mixers and decanters and         a heat exchanger containing glycol as heat-transfer fluid.

At the end of 30 hours of operation, samples of the organic phases and of the aqueous phases are taken to quantify the concentration profiles on each of the 3 stages as well as the yield of iron removal (de-ironing yield) from the balance on the organic phases.

The results of these tests are presented in Table III below.

TABLE III Concentration profiles of uranium and iron in each MD stage. Stage 1 (*) Stage 2 Stage 3 (**) Influent Contact: 7.5 min Contact: 7.5 min Contact: 12.5 min Aqueous phase [U] (g/L)   39.76 0.68 21.82 33.00 [Fe] (mg/L) 0 328 116 22 Organic phase [U] (g/L)    0.994 22.13 30.90 38.94 [Fe] (mg/L) 307  114 32 5 Iron removal — 62.9% 89.6% 98.2% Fe/U by weight ~31%  0.515% 0.104% 0.014% (*) Organic phase supply in stage 1 (**) Aqueous phase supply in stage 3

Example 4

In this example, solvents or extraction systems loaded with uranium are prepared by placing the attack liquors loaded with uranium in contact with solvents.

In a first step, a sulphuric attack liquor or solution, a phosphoric attack liquor or solution, and a nitric attack liquor or solution are prepared.

The phosphoric and sulphuric attack liquors are prepared from industrial liquors, juices.

Thus, the sulphuric attack liquor comes from an ore lixiviation liquor, juice doped with vanadium and with zirconium (see Table IV).

TABLE IV Composition and characterisation of the sulphuric attack liquor Element Concentration (mg/L) Experimental data U 4,430.0 Specific gravity (20° C.) 1.102 Fe 8,187.9 [H*] (M) 0.06 Mo 471.7 E_(h) (mV) (Ref. Ag/AgCl) 505 V 1,179.1 Zr 3,085.6

The phosphoric attack liquor comes from an American industrial phosphoric acid SIMPLOT diluted twice and doped with uranium (see Table V).

TABLE V Composition and characterisation of the phosphoric attack liquor Element Concentration (mg/L) Experimental data U 1,239.1 Specific gravity (20° C.) 1.158 Fe 2,802.4 [H*] (M) 0.96 Mo 4.7 E_(h) (mV) (Ref. Ag/AgCl) 490 V 262.9 Zr 15.1

The nitric attack liquor was, for its part, prepared from nitric acid, uranyl nitrate and iron sulphate (III) (see Table VI).

TABLE VI Composition and characterisation of the nitric attack liquor Element Concentration (mg/L) Experimental data U 4,698.5 Specific gravity (20° C.) 1.163 Fe 4,500.8 [H*] (M) 0.20 E_(h) (mV) (Ref. Ag/AgCl) 695

The chemical analysis of the solutions shows that the sulphuric solution is highly loaded with impurities, notably iron, vanadium and zirconium.

On the other hand, the phosphoric and nitric solutions are only loaded with iron. Furthermore, the redox potential of the solutions thereby prepared shows that iron is mainly in ferric form (the redox potential of the solution being inherent to the pair iron(II)/iron(III)).

It should be recalled that the partition coefficient of iron(III) is higher than that of iron(II) on organophosphorous solvents.

In a second step, the solvents are prepared.

These solvents comprise an organic organophosphorous extractant or a mixture of organic organophosphorous extractant(s), diluted in an organic diluent, non-reactive and non-miscible with water, namely an aliphatic kerosene (ISANE® IP 185).

The following extractants were chosen:

-   -   D₂EHPA (di-2-ethylhexylphosphoric acid) supplied by the Lanxess         company.     -   TOPO (trioctyl phosphine oxide) supplied by the Cytec company.     -   TBP (tributyl phosphate).     -   DEHCNPB (Butyl 1-(Diethylhexyl Carbamoyl) Nonyl Phosphonate).

With these extractants and the diluent ISANE® IP 185, the following 5 solvents were prepared:

0.5 M D₂EHPA in ISANE®.

0.5 M D₂EHPA+0.125 M TOPO (solvent of the Oak Ridge method) in ISANE.

0.2 M D₂EHPA+0.2 M TBP (solvent of the DAPEX method) in ISANE.

0.1 M or 0.5 M DEHCNPB in ISANE.

In a third step, the solvents described above are contacted with the attack liquors prepared beforehand during the first step. The placing in contact is carried out for 30 minutes at room temperature (25° C.) with an O/A phase volume ratio of 1/1, the volume of the aqueous phase A and the organic phase each being 100 ml.

The various contacts tests carried out during this third step are described in table VII.

TABLE VII Description of the tests selected for this study Experiment Solvent Attack liquor A D₂EHPA 0.5M + TOPO 0.125M Sulphuric B D₂EHPA 0.5M C DEHCNPB 0.5M D DEHCNPB 0.1M E D₂EHPA 0.2M + TBP 0.2M F DEHCNPB 0.5M Phosphoric G DEHCNPB 0.1M H D₂EHPA 0.2M + TBP 0.2M L D₂EHPA 0.5M + TOPO 0.125M I D₂EHPA 0.5M + TOPO 0.125M Nitric J DEHCNPB 0.5M K D₂EHPA 0.5M M D₂EHPA 0.2M + TBP 0.2M

An analysis of the loaded solvents at the end of these first contacts was conducted with monitoring of the concentrations of uranium, iron, molybdenum, vanadium and zirconium (see Tables VIII to X).

It should be noted that problems of analytical reproducibility (mineralisation of the solvent and associated acquisition) did not make it possible to determine the concentration of zirconium in the loaded solvents. The trends will thus be drawn up on the basis of monitoring in aqueous phase for this element if need be.

TABLE VIII Composition of the solvents loaded from the sulphuric attack liquor Test A B C D E Solvent D₂EHPA 0.5M D₂EHPA 0.5M DEHCNPB 0.5M DEHCNPB 0.1M D₂EHPA 0.2M TOPO 0.125M TBP 0.2M Specific gravity (20° C.) 0.812 0.804 0.825 0.779 0.792 [U] (mg/L) 3,032.8 2,665.0 2,482.4 3,781.3 3,235.3 [Fe] (mg/L) 1,177.4 1,234.2 1,122.0 246.9 407.9 [Mo] (mg/L) 289.3 218.4 253.3 177.6 295.4 [V] (mg/L) 203.8 162.4 344.9 30.4 137.8 [Zr] (mg/L)

The loaded solvents derived from the contacts with the sulphuric solution are globally loaded at 3 g/L of uranium, between 0.5 and 1 g/L of iron, 200 mg/L of molybdenum and vanadium.

It may be noted that the observed extraction performances between the systems 0.5 M DEHNCPB and 0.5 M D₂EHPA are similar in the studied sulphuric medium.

Furthermore, it would seem that a reduction in the concentration of DEHCNPB leads to better uranium/impurities selectivity.

TABLE IX Composition of the solvents loaded from the phosphoric attack liquor Test F G H L Solvent DEHCNPB 0.5M DEHCNPB 0.1M D₂EHPA 0.2M D₂EHPA 0.5M TBP 0.2M TOPO 0.125M Specific gravity (20° C.) 0.822 0.776 0.787 0.804 [U] (mg/L) 938.7 951.4 780.7 918.4 [Fe] (mg/L) 1,537.1 274.7 61.4 699.7 [Mo] (mg/L) 3.5 2.4 <1 2.7 [V] (mg/L) 132.3 30.3 1.5 16.9 [Zr] (mg/L)

The loaded solvents derived from the contacts with the phosphoric solution are globally loaded with 1 g/L of uranium with variable concentrations of impurities, notably for iron and vanadium.

Molybdenum, for its part, is very little loaded in all of the solvents; this being linked to the very low initial concentration of molybdenum in the phosphoric liquor and/or to the highly complexing nature of this matrix.

Thus, the mixture D₂EHPA/TBP seems to be the system the most selective to extraction in our conditions since the concentrations of iron, molybdenum and vanadium are very low.

Furthermore, a gain in selectivity is also obtained when the concentration of DEHCNPB is reduced.

TABLE X Composition of the solvents loaded from the nitric attack liquor Test I J K M Solvent D₂EHPA 0.5M DEHCNPB 0.5M D₂EHPA 0.5M D₂EHPA 0.2M TOPO 0.125M TBP 0.2M Specific gravity (20° C.) 0.814 0.827 0.805 0.793 [U] (mg/L) 3,545.8 3,344.4 3,640.2 3,556.6 [Fe] (mg/L) 3,231.6 3,374.2 3,115.4 1,498.8

The solvents derived from the contacts with the nitric solution are globally loaded with 3 g/L of uranium and iron, apart from the system D₂EHPA/TBP for which the concentration of iron is two times lower.

Furthermore, similar performances between the molecules D₂EHPA and DEHCNPB are observed and it is noted that TOPO no longer here plays the role of synergy agent in a nitric matrix.

Example 5

In this example, aqueous solutions are prepared, referred to as aqueous de-ironing solutions, intended to be contacted with the loaded solvents prepared in example 4 with the aim of separating iron from these loaded organic solvents.

Two aqueous solutions are prepared (see Table XI): namely a 1.5 M solution of pure sulphuric acid (which does not comply with the aqueous de-ironing solution used in the method of the invention) which constitutes the reference aqueous solution, and a 1.5 M sulphuric acid solution containing uranium at a level of 40 g/L (in accordance with the aqueous de-ironing solution used in the method of the invention) which constitutes the aqueous solution under study.

The tests carried out with the reference solution (pure acid) will be identified by the number 1 following the letter designating the loaded solvent contacted with the aqueous solution, whereas the tests carried out with the solution in accordance with the method according to the invention will be identified by the number 2 following the letter designating the loaded solvent contacted with the aqueous solution.

TABLE XI Composition of the aqueous reagents used for the scrubbing of impurities from the loaded solvents Solution Specific gravity (20° C.) [H₂SO₄] (M) [U] (mg/L) Reference 1.093 1.57 <1 Study 1.146 1.54 40,454

Example 6

In this example, purification tests are carried out on loaded solvents prepared in example 4 from the sulphuric attack liquor (Table VIII, tests A to E), by means of the aqueous solution in accordance with that used in the method according to the invention and of the comparative solution prepared in example 5.

The conditions for these tests are the following:

-   -   Use of dedicated separating funnels and mechanical stirrers.     -   Duration: 30 minutes.     -   Room temperature (25° C.).     -   Volume ratio of the O/A phases of 1/1 (80 ml for each of the         phases).

At the end of the contacts between the loaded solvents and the 1.5 M solution of sulphuric acid containing or not uranium, analyses are conducted on the aqueous phases (cf. tables XII and XIV) and the organic phases (cf. tables XIII and XV) after filtration with monitoring of uranium, and iron, molybdenum, vanadium and zirconium impurities.

As a reminder, the analytical uncertainty is comprised between 5 and 10% depending on the considered element.

TABLE XII Analyses of the aqueous phases after contact of the loaded solvents with a pure 1.5M sulphuric acid solution Test A1 B1 C1 D1 E1 Specific 1.093 1.093 1.092 1.091 1.091 gravity (20° C.) [U]_(aq) (mg/L) 2.0 70.0 <1 1.3 182.2 [Fe]_(aq) (mg/L) 170.5 31.7 55.7 185.5 86.2 [Mo]_(aq) (mg/L) 1.6 2.2 <1 10.6 20.7 [V]_(aq) (mg/L) 192.4 158.5 203.1 32.7 141.8 [Zr]_(aq) (mg/L) 67.8 5.1 <1 4.6 159.3

TABLE XIII Analyses of the organic phases after contact of the loaded solvents with a 1.5M sulphuric acid solution Test A1 B1 C1 D1 E1 Specific 0.811 0.804 0.825 0.779 0.792 gravity (20° C.) [U]_(orga) (mg/L) 3,167.8 2,713.3 2,659.8 3,804.4 3,169.6 [Fe]_(orga) (mg/L) 1,013.8 1,196.5 957.0 54.5 343.4 [Mo]_(orga) (mg/L) 287.9 219.2 251.6 167.3 264.1 [V]_(orga) (mg/L) 13.0 8.8 146.9 1.2 1.3 [Zr]_(orga) (mg/L)

As a reminder, the calculated material balance holds globally for all of the analysed elements to ±5%.

The results of the tests carried out with an aqueous solution constituted by pure sulphuric acid which are set out in Tables XII and XIII show that a washing with pure 1.5 M sulphuric acid makes it possible:

-   -   to limit the loss in uranium except for tests B and E for which         the concentration of uranium in the aqueous phase is greater         than 50 mg/L;     -   to ensure good scrubbing of the solvent of vanadium, apart from         test C; which complies with the literature data;     -   to eliminate only a small proportion of the iron contained in         the loaded solvents.

Washing tests are then carried out on loaded solvents, with a sulphuric solution of same acidity (1.5 M) but containing uranium, that is to say a solution in accordance with that used in the method according to the invention.

The results of these tests are set out in Tables XIV and XV.

TABLE XIV Analyses of the aqueous phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test A2 B2 C2 D2 E2 Specific gravity (20° C.) 1.115 1,121 1.110 1.135 1.143 [U]_(aq) (mg/L) 8,552.1 14,685.1 2,064.6 30,304.5 28,575.0 [Fe]_(aq) (mg/L) 1,181.9 854.2 1,024.5 278.1 418.3 [Mo]_(aq) (mg/L) 27.9 10.6 26.6 128.3 147.4 [V]_(aq) (mg/L) 199.6 164.8 316.9 28.4 137.2 [Zr]_(aq) (mg/L) 399.2 265.7 542.8 535.7 480.1

TABLE XV Analyses of the organic phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test A2 B2 C2 D2 E2 Specific gravity (20° C.) 0.845 0.830 0.866 0.787 0.803 [U]_(orga) (mg/L) 34,302.8 29,109.8 44,881.3 12,197.7 14,374.5 [Fe]_(orga) (mg/L) 65.1 356.9 116.0 8.7 48.2 [Mo]_(orga) (mg/L) 253.5 215.0 226.9 59.0 161.4 [V]_(orga) (mg/L) 8.4 2.2 <1 1.3 1.3 [Zr]_(orga) (mg/L)

As a reminder, once again, the calculated material balance holds globally for all of the elements analysed to ±5%.

The results of the tests show that a washing with pure 1.5 M sulphuric acid containing uranium, in accordance with the method of the invention, makes it possible:

-   -   to scrub in a limited manner molybdenum but with yields greater         than those observed with a solution of sulphuric acid free of         uranium;     -   to ensure quasi-quantitative scrubbing of the solvent of         vanadium, including for test C (unlike the solution of sulphuric         acid free of uranium);     -   to eliminate quasi-quantitatively the iron contained in the         loaded solvent for all of the tests;     -   to improve the elimination of zirconium if based uniquely on the         analyses of the aqueous phases after contact.

Example 7

In this example, purification tests are carried out on the loaded solvents prepared in example 4 from the phosphoric attack liquor (Table IX, tests F to H and L), by means of the aqueous solution in accordance with that used in the method according to the invention and of the comparative solution prepared in example 5.

The conditions of these tests are the following:

-   -   Use of dedicated separating funnels and mechanical stirrers.     -   Duration: 30 minutes.     -   Room temperature (25° C.).     -   Volume ratio of the O/A phases of 1/1 (80 ml for each of the         phases).

At the end of the contacts between the loaded solvents and the 1.5 M sulphuric acid solution containing or not uranium, analyses are carried out on the aqueous phases (cf. tables XVI and XVIII) and on the organic phases (cf. tables XVII and XIX) after filtration with monitoring on uranium and iron, molybdenum, vanadium and zirconium impurities.

As a reminder, the analytical uncertainty is comprised between 5 and 10% depending on the considered element.

TABLE XVI Analyses of the aqueous phases after contact of the loaded solvents with a pure 1.5M sulphuric acid solution Test F1 G1 H1 L1 Specific gravity (20° C.) 1.087 1.091 1.089 1.093 [U]_(aq) (mg/L) <1 1.9 13.1 1.9 [Fe]_(aq) (mg/L) 75.0 93.8 4.7 56.8 [Mo]_(aq) (mg/L) <1 <1 <1 <1 [V]_(aq) (mg/L) 67.4 28.4 1.5 15.3 [Zr]_(aq) (mg/L) <1 <1 <1 <1

TABLE XVII Analyses of the organic phases after contact of the loaded solvents with a pure 1.5M sulphuric acid solution Test F1 G1 H1 L1 Specific gravity (20° C.) 0.821 0.775 0.787 0.806 [U]_(orga) (mg/L) 939.2 951.7 785.4 905.1 [Fe]_(orga) (mg/L) 1,494.2 185.2 55.1 689.9 [Mo]_(orga) (mg/L) 3.4 2.5 <1 2.6 [V]_(orga) (mg/L) 66.8 3.3 <1 1.3 [Zr]_(orga) (mg/L)

As a reminder, the calculated material balance holds globally for all of the elements analysed to ±5%.

The results of the tests carried out with an aqueous solution constituted by pure sulphuric acid which are set out in Tables XVI and XVII show that a washing with pure 1.5 M sulphuric acid makes it possible:

-   -   to limit the loss in uranium except for test H for which the         concentration of uranium in the aqueous phase is greater than 10         mg/L;     -   to ensure quasi-quantitative scrubbing of the solvent of         vanadium, apart from test F;     -   to eliminate only a small proportion of iron contained in all         the loaded solvents studied.

Washing tests are then carried out of the loaded solvents, with a sulphuric solution of same acidity (1.5 M) but containing uranium, that is to say a solution in accordance with that used in the method according to the invention.

The results of these tests are set out in Tables XVIII and XIX.

TABLE XVIII Analyses of the aqueous phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test F2 G2 H2 L2 Specific 1.110 1.135 1.130 1.110 gravity (20° C.) [U]_(aq) (mg/L) 888.8 27,467.0 25,199.0 6,682.2 [Fe]_(aq) (mg/L) 1,430.0 252.0 44.1 653.8 [Mo]_(aq) (mg/L) <1 <1 <1 <1 [V]_(aq) (mg/L) 129.8 25.0 <1 12.2 [Zr]_(aq) (mg/L) 2.6 2.2 <1 <1

TABLE XIX Analyses of the organic phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test F2 G2 H2 L2 Specific 0.864 0.788 0.803 0.842 gravity (20° C.) [U]_(orga) (mg/L) 42,240.1 13,285.7 15,602.3 34,566.6 [Fe]_(orga) (mg/L) 56.2 15.0 17.7 18.5 [Mo]_(orga) 4.8 1.0 <1 2.5 (mg/L) [V]_(orga) (mg/L) <1 <1 <1 2.5 [Zr]_(orga) (mg/L)

As a reminder, once again, the calculated material balance holds globally for all of the elements analysed to ±5%.

The results of the tests show that a washing with pure 1.5 M sulphuric acid containing uranium, in accordance with the method of the invention, makes it possible:

-   -   to ensure quasi-quantitative scrubbing of the solvent of         vanadium;     -   to eliminate quasi-quantitatively iron from all the loaded         solvents studied.

Example 8

In this example, purification tests are carried out of the loaded solvents prepared in example 4 from the nitric attack liquor (Table X, tests I to K and M), by means of the aqueous solution in accordance with that used in the method according to the invention and of the comparative solution prepared in example 5.

The conditions for these tests are the following:

-   -   Use of dedicated separating funnels and mechanical stirrers.     -   Duration: 30 minutes.     -   Room temperature (25° C.).     -   Volume ratio of the O/A phases of 1/1 (80 ml for each of the         phases).

At the end of the contacts between the loaded solvents and the 1.5 M sulphuric acid solution containing or not uranium, analyses are conducted on the aqueous phases (cf. tables XX and XXII) and the organic phases (cf. tables XXI and XXIII) after filtration with monitoring of uranium and iron as main impurity.

As a reminder, the analytical uncertainty is comprised between 5 and 10% depending on the considered element.

TABLE XX Analyses of the aqueous phases after contact of the loaded solvents with a pure 1.5M sulphuric acid solution Test I1 J1 K1 M1 Specific gravity (20° C.) 1.093 1.088 1.089 1.095 [U]_(aq) (mg/L) 2.5 <1 291.9 146.7 [Fe]_(aq) (mg/L) 589.1 256.8 369.2 734.7

TABLE XXI Analyses of the organic phases after contact of the loaded solvents with a pure 1.5M sulphuric acid solution Test I1 J1 K1 M1 Specific gravity (20° C.) 0.813 0.827 0.803 0.791 [U]_(orga) (mg/L) 3,579.6 3,384.9 3,422.4 3,383.1 [Fe]_(orga) (mg/L) 2,658.5 3,059.9 2,762.3 739.6

As a reminder, the calculated material balance holds globally for all of the elements analysed to ±5%.

The results of the tests carried out with an aqueous solution constituted by pure sulphuric acid which are set out in Tables XX and XXI show that a washing with pure 1.5 M sulphuric acid makes it possible:

-   -   to limit the loss in uranium except for tests K and M for which         the concentration of uranium in the aqueous phase is greater         than 100 mg/L;     -   to only eliminate a very small proportion of iron contained in         the loaded solvents, apart from test M.

Washing tests are next carried out on the loaded solvents with a sulphuric solution of same acidity (1.5 M) but containing uranium, that is to say a solution in accordance with that used in the method according to the invention.

The results of these tests are set out in Tables XXII and XXIII.

TABLE XXII Analyses of the aqueous phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test I2 J2 K2 M2 Specific 1.113 1.113 1.123 1.135 gravity (20° C.) [U]_(aq) (mg/L) 6,499.9 617.7 14,599.0 26,672.5 [Fe]_(aq) (mg/L) 2,916.1 3,238.8 2369.5 1,418.8

TABLE XXIII Analyses of the organic phases after contact of the loaded solvents with a 1.5M sulphuric acid solution containing uranium Test I2 J2 K2 M2 Specific 0.847 0.867 0.829 0.806 gravity (20° C.) [U]_(orga) (mg/L) 35,133.6 41,284.8 29,509.9 16,929.2 [Fe]_(orga) (mg/L) 155.8 124.8 457.6 23.4

As a reminder, again, the calculated material balance holds globally for all of the elements analysed, to ±5%.

The results of the tests show that a washing with pure 1.5 M sulphuric acid containing uranium, in accordance with the method of the invention, makes it possible to eliminate quasi-quantitatively in one contact the iron contained in the loaded solvents for all of the tests with very good yields for tests I, J, and M.

Conclusions from Examples 6, 7, and 8

The yields of iron removal are represented in FIG. 3 for all of the tests of examples 6, 7, and 8 as a function of the aqueous iron removal solutions used, namely the pure 1.5 M sulphuric acid solution (which does not comply with the aqueous iron removal solution used in the method of the invention) which constitutes the reference aqueous solution, and the 1.5 M sulphuric acid solution containing uranium in an amount of 40 g/L, which complies with the aqueous iron removal solution used in the method of the invention).

The change in these yields (FIG. 3) clearly shows a significant gain when the washing of the loaded solvent is carried out in the presence of uranium for all of the tests.

Indeed, the yields of iron removal obtained in one contact are generally less than 20% in the case of the reference aqueous solution (pure solution of sulphuric acid), apart from tests D and M with yields of iron removal of the order of 80 and 50% respectively.

The yields of iron removal obtained in one contact are 90% in the case of the uranium bearing solution of sulphuric acid, apart from tests B and H for which the yield is of the order of 70%.

These results clearly show that the method according to the invention, which uses an aqueous de-ironing (iron removal) solution containing uranium, may be implemented with success with all the loaded solvents, whether said solvents contain pure organophosphorous extractants or as synergistic mixtures and whatever the matrix of the attack liquor having made it possible to obtain these loaded solvents. 

What is claimed is:
 1. Method for separating iron from an initial liquid organic phase containing uranium and iron, wherein the initial liquid organic phase is contacted with an aqueous solution referred to as aqueous de-ironing solution, whereby the iron passes into the aqueous solution to form a final liquid aqueous phase, and uranium remains in the initial liquid organic phase to form a final liquid organic phase referred to as de-ironed organic phase; said method being characterised in that the aqueous de-ironing solution contains an inorganic acid and uranium, and does not contain iron.
 2. Method according to claim 1, wherein the initial liquid organic phase comprises an organic extraction system comprising an organic extractant or a mixture of organic extractant(s), diluted in an organic diluent, non-reactive and non-miscible with water.
 3. Method according to claim 2, wherein the organic extraction system comprises an extractant selected from organophosphorous compounds and mixtures thereof.
 4. Method according to claim 3, wherein the organic extraction system comprises an extractant selected from acid organophosphorous compounds such as dialkylphosphoric acids, bifunctional organophosphorous compounds, neutral phosphine oxides such as trialkylphosphine oxides, and mixtures thereof.
 5. Method according to claim 4, wherein the extraction system comprises the mixture of an acid organophosphorous compound and of a neutral phosphine oxide.
 6. Method according to claim 2, wherein, the extractant system comprises as extractant a compound which corresponds to the following general formula (I):

wherein: m is a whole number equal to 0, 1 or 2; R¹ and R², identical or different, are a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms; R³ is: a hydrogen atom; a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 1 to 12 carbon atoms and optionally one or more heteroatoms; a monocyclic hydrocarbon group, saturated or unsaturated, comprising from 3 to 8 carbon atoms and optionally one or more heteroatoms; or a monocyclic aryl or heteroaryl group; or instead R¹ and R³ together form a —(CH₂)_(n)— group wherein n is a whole number ranging from 1 to 4; R⁴ is a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 2 to 8 carbon atoms, or a monocyclic aromatic group; whereas R⁵ is a hydrogen atom or a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 1 to 12 carbon atoms.
 7. Method according to claim 6, wherein the compound of formula (I) corresponds to the following specific formula (I-a):

wherein: m, R⁴ and R⁵ are as defined in claim 6; R² is a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 6 to 12 carbon atoms; whereas R³ is: a hydrogen atom; a hydrocarbon group, saturated or unsaturated, linear or branched, comprising from 1 to 12 carbon atoms and optionally one or more heteroatoms; a monocyclic hydrocarbon group, saturated or unsaturated, comprising from 3 to 8 carbon atoms and optionally one or more heteroatoms; or a monocyclic aryl or heteroaryl group.
 8. Method according to claim 6, wherein the compound of formula (I) corresponds to the specific formula (I-b):

wherein m, n, R⁴ and R⁵ are such as defined in claim
 6. 9. Method according to claim 1, wherein the initial organic phase contains from 0.5 g/L to 10 g/L of uranium and from 0.1 to 10 g/L of iron.
 10. Method according to claim 1, wherein the inorganic acid of the aqueous de-ironing solution is selected from sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid, and mixtures thereof.
 11. Method according to claim 10, wherein the inorganic acid of the aqueous de-ironing solution is sulphuric acid.
 12. Method according to claim 1, wherein the quantity of uranium provided by the aqueous de-ironing solution is such that the concentration of uranium in the organic phase is at least equal to 50%, preferably at least equal to 60%, further preferably at least equal to 70%, of the concentration of uranium corresponding to saturation of the organic phase with uranium.
 13. Method according to claim 1, wherein the concentration of uranium, expressed in [U], of the aqueous de-ironing solution is from 0.10 to 800 g/L, preferably from 30 to 50 g/L, for example 40 g/L.
 14. Method according to claim 1, wherein, during the contacting the initial organic phase is mixed with the aqueous de-ironing solution then said mixture is decanted, wherein the contacting is carried out in a battery of 1 to 5 mixers-decanters, counter-current supplied with the initial organic phase and with the aqueous de-ironing solution.
 15. Method according to claim 1, wherein the final aqueous phase contains more than 90% of the weight of iron contained in the initial organic phase, and less than 1% of the weight of uranium contained in the initial organic phase, and the de-ironed organic phase contains at least 90% of the weight of uranium contained in the initial organic phase, and less than 10% of the weight of iron contained in the initial organic phase.
 16. Method for extracting uranium from a first aqueous solution of an inorganic acid containing uranium and iron, wherein at least the following successive steps are carried out: a) the first aqueous solution of inorganic acid is contacted with a first liquid organic phase; whereby are obtained, on the one hand, a second liquid organic phase containing a majority by weight of the quantity of uranium contained in the first aqueous solution of inorganic acid, and a minority by weight of the quantity of iron contained in the first aqueous solution of inorganic acid and, on the other hand, a second desuraniated aqueous phase containing the inorganic acid, a minority by weight of the quantity of uranium contained in the aqueous solution of inorganic acid, and a majority by weight of the quantity of iron contained in the aqueous solution of inorganic acid; b) the iron is separated from the second liquid organic phase containing uranium and iron, by contacting the second liquid organic phase with a third aqueous solution referred to as aqueous de-ironing solution, whereby the iron passes into the aqueous de-ironing solution to form a final liquid aqueous phase, and uranium remains in the second liquid organic phase to form a final liquid organic phase referred to as de-ironed organic phase; said method being characterised in that the aqueous de-ironing solution contains an inorganic acid and uranium, and does not contain iron.
 17. Method according to claim 16, wherein the inorganic acid of the first aqueous solution of inorganic acid of step a) is a solution of phosphoric acid, sulphuric acid or nitric acid.
 18. Method according to claim 16, wherein the first aqueous solution of inorganic acid of step a) contains from 0.1 to 10 g/L of iron, and from 0.05 to 10 g/L of uranium.
 19. Method according to claim 16, wherein the first aqueous solution of inorganic acid is an aqueous uranium bearing solution of phosphoric acid, such as industrial phosphoric acid, coming from the lixiviation, attack, of a natural phosphate ore, generally based on apatite, by sulphuric acid, or an aqueous uranium bearing solution of sulphuric acid or nitric acid coming from the lixiviation, attack, of a non-phosphate uranium bearing ore, for example non-apatite based, respectively by sulphuric acid or nitric acid.
 20. Method according to claim 16, wherein, in step a), the second organic phase obtained contains at least 90% by weight, for example from 95 to 100% by weight, of the quantity of uranium contained in the first aqueous solution of inorganic acid (starting solution), and from 0.1 to 50% by weight of the quantity of iron contained in the first aqueous solution of inorganic acid; and the second desuraniated aqueous phase obtained contains the inorganic acid, from 0 to 10% by weight of the quantity of uranium, and from 50 to 99.9%, for example from 80% to 90% by weight of the quantity of iron contained in the first aqueous solution of inorganic acid (starting solution).
 21. Method according to claim 16, wherein the second organic phase obtained at the end of step a) contains from 0.5 to 10 g/L of uranium and from 0.1 to 10 g/L of iron, and the second aqueous phase obtained at the end of step a) contains from 0 to 100 mg/L of uranium and from 0.1 to 6 g/L of iron.
 22. Method according to claim 16, which further comprises a step c) wherein the iron-removed organic phase obtained in step b) is contacted with an aqueous solution of a complexing base; whereby are obtained, on the one hand, an aqueous phase loaded with uranium and, on the other hand, an organic phase free of uranium, and further containing said complexing base.
 23. Method according to claim 22, which further comprises a step d) wherein the organic phase free of uranium, further containing the complexing base obtained in step c), is contacted with the aqueous phase coming from step b) and neutralised, whereby are obtained, on the one hand, an organic phase consisting of the organic solvent which is sent back to step a) and, on the other hand, an aqueous phase.
 24. Method according to claim 22, which further comprises a step e) wherein the aqueous phase loaded with uranium obtained in step c), is contacted with a base such as sodium hydroxide, whereby an uranate precipitate, such as sodium uranate precipitate, which is separated, and an aqueous solution which is sent to step c) after addition of a complexing base, are obtained.
 25. Method according to claim 24, wherein all or part of the uranate precipitate, such as sodium uranate, obtained in step e), is dissolved in an inorganic acid such as sulphuric acid, and the aqueous solution obtained containing an inorganic acid and uranium is sent to step b) after having optionally adjusted the concentration of inorganic acid. 